Zeolites, particularly type X zeolite, having lithium as the major exchangeable cation have been shown to adsorb nitrogen from gas streams much more strongly than do zeolites which have other cations, e.g. sodium ions, as the major exchangeable cation. See, for example, U.S. Pat. Nos. 3,140,933 and 4,859,217, which disclose the use of type X zeolite having 86% and greater than 88%, respectively, of its base ions replaced by lithium ions for the separation of nitrogen from oxygen. Lithium-exchanged natural mordenite is reported to be a good adsorbent for oxygen pressure swing adsorption (PSA) in papers by H. Minato and M. Watanabe, published in Scientific Paper, University of Tokyo, (1978), 28, 218, and S. Furuyama and K. Sato in the Journal of Physical Chemistry (1982), 86, 2498-2503.
Zeolites with high percentages of lithium ions as charge compensating ions have very high affinities for water, and adsorbed water, even in small amounts, seriously diminishes the adsorptive capacity of the zeolites. Accordingly, to secure optimum adsorption performance, it is necessary to activate these zeolites by heating them to temperatures as high as 600 to 700.degree. C. to drive off as much adsorbed water as possible. Care must be exercised during activation, however, to avoid heating these zeolites to temperatures much above 700.degree. C., since highly lithium ion-exchanged zeolites have reduced thermal stability at such high temperatures.
U.S. Pat. No. 5,174,979 asserts that lithium/alkaline earth metal X zeolites having lithium/alkaline earth metal molar ratios in the range of about 95:5 to 50:50 have thermal stabilities greater than the corresponding pure lithium zeolites and good adsorption capacities and selectivities, and U.S. Pat. No. 5,152,813 discloses the adsorption of nitrogen from gas mixtures using crystalline X-zeolites having as exchangeable ions combinations of lithium and a second ion selected from calcium, strontium and mixtures of these.
U.S. Pat. No. 5,464,467 discloses the preparation of nitrogen-selective lithium-and trivalent ion-exchanged type X zeolites that have improved thermal stability relative to comparable lithium-exchanged type X zeolites. The exchanged zeolites were prepared by pelletizing (without a binder) type X zeolite or type LSX (low silicon type X zeolite), then exchanging the pelletized zeolites with aqueous lithium chloride solution and finally exchanging the lithium-exchanged zeolites with selected lanthanide chlorides, or by first exchanging sodium LSX with an aqueous rare earth chloride solution and then lithium exchanging the rare earth-exchanged zeolite to the final product without an intermediate calcination step. The disclosure of this patent is incorporated herein by reference.
U.S. patent application Ser. No. 08/515,184, filed Aug. 11, 1995, now U.S. Pat. No. 5,616,170 discloses the preparation and use of thermally stable nitrogen-selective lithium and trivalent ion-exchanged zeolites selected from chabazite, offretite, erionite, levyne mordenite, gmelinite, zeolite A, zeolite T, EMC-2, ZSM-3, ZSM-18, ZK-5, zeolite L, beta zeolite, and mixtures of these. The trivalent ions which are exchanged with the zeolite are selected from aluminum, scandium, gallium, indium, yttrium, iron (III), chromium (III), single lanthanides, mixtures of two or more lanthanides and mixtures of these. It is stated in the disclosure of this patent application that the order of cation exchange is not critical, but it is preferred to first cation exchange the base zeolite with one or more of the specified trivalent ions and then exchange the zeolite with lithium ions. It is further stated in this patent that aggregation can be effected before, after or between the trivalent metal and lithium ion-exchanges but it is preferred to use as the starting material an X zeolite which is already in aggregate form because the sodium form of the zeolite is more thermally stable than the lithium/trivalent metal zeolites and less susceptible to damage from the high temperatures used in the aggregation process. The disclosure of this patent application is incorporated herein by reference.
It has been discovered that when an aggregated form of the zeolite is ion-exchanged with a trivalent cation, the trivalent cation is concentrated at or near the surface of the aggregated particles. It appears that the trivalent cations are not able to penetrate deeply into the particles to produce particles having a uniform distribution of trivalent cations throughout their interior portions. It is highly desirable to produce exchanged zeolite particles which have a uniform distribution of trivalent cations because portions of the zeolite which are rich in lithium ions but depleted in trivalent cations tend to undergo degradation at the high temperatures required for activation of the zeolite. Accordingly, methods of preparing trivalent cation- and lithium ion-exchanged zeolites with uniform distribution of both the trivalent cations and the lithium ions are desired for commercial manufacturing purposes. The present invention provides such a method.